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"This and these have the Ts" covers part of it... "esto and estos" are "this and that"... not sure how much that helps :)
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Flimzy♦Apr 26 '14 at 19:02

Did you consult a dictionary? Of course you need memorization: you cannot just guess. Why did you think that esos could be translated as these? You don't need a rule of thumb. You just need to learn or look up those words in a dictionary.
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c.p.Apr 29 '14 at 20:35

Eso and esto are not masculine. They are neuter pronouns, never articles, and never have accent. The masculine pronoun for that is ése. The masculine article and pronoun for this are este and éste.
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rsanchezApr 26 '14 at 20:14

Are the forms with "tilde" (éste, ése etc.) still mandatory? I heard that one can now use the forms without "tilde" as pronouns.
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Alfonso VillénMay 4 '14 at 20:53

Este and Esta are used, when referring to nearby object, depending on the gender of that object.

Quien bebe de esta taza?

can never be "esto" because taza is feminine (la taza).

Now, when you say "These birds are not black" you are referring to birds that are near. But in your example you have to translate:

Esos pajaros no son negros.

which corresponds to birds, that are not that close - "those birds".

With the second example:

Ese vaso es grande.

Ese is correct here, because vaso is masculine (el vaso). Eso is neuter and is normally used when the object was already mentioned, or when the gender cannot be determined or when describing an action. Like here:

Welcome to Spanish.SE! We're glad you're here. But this doesn't seem to me really to answer the question which was asked, which is how to remember when to use and when not to use the accent marks.
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Flimzy♦Apr 29 '14 at 21:19